Many building structure designs include the use of diagonal braces to provide lateral stability, especially for the purpose of increasing the lateral stiffness of the structure and reducing the cost of construction. In such bracing systems it is known that one or more sacrificial yielding fuse elements may be implemented in order to dissipate seismic input energy in the event of dynamic loading, such as during a severe seismic event. Such sacrificial yielding fuse elements can be designed to provide improved seismic performance and reduced seismic loads when compared to traditional lateral load resisting systems.
Many prior art systems require a buckling restraining apparatus used in conjunction with a yielding member, and make use of axially yielding members, whereas it would be advantageous to use flexural yielding elements as they are less prone to fracture caused by excessive inelastic straining.
U.S. Pat. No. 4,823,522 to White, U.S. Pat. No. 4,910,929 to Scholl and U.S. Pat. No. 5,533,307 to Tsai and Li all describe steel yielding fuse elements that are placed at the centre of a beam and are used to add damping and stiffness to a seismically loaded moment resisting frame. The damping elements are generally formed with steel plates that are cut into triangular shapes and welded or bolted to a rigid base. Also, these elements are generally installed at the centre of the upper brace in and inverted V-type braced frame. Thus the yielding of these elements is controlled by the inter-story displacement of the frame. However, a yielding element that was linked to the brace elongation rather than the inter-story displacement would integrate more easily with current construction practices.
Another prior art fuse system, the EaSy Damper, uses a complex fabricated device to improve the seismic performance of brace elements by replacing axial yielding and buckling of the brace with combined flexural and shear yielding of a perforated, stiffened steel plate. The shapes of these plates do not result in constant curvature of the yielding elements and thus lead to undesirable strain concentrations.
Both of the aforementioned prior art systems require painstaking cutting and welding fabrication. Furthermore, the limited geometry of currently available rolled steel products restricts the potential geometry of the critical yielding elements of such devices.
Having greater control of the geometry of the flexural yielding elements permits control of not only the force at which the fuse yields, but also the elastic and post yield stiffnesses of the fuse as well as the displacement associated with the onset of fuse yielding. With casting technology a better performing fuse can be designed and manufactured. Also, free geometric control would enable the design of a part that would more easily integrate with existing steel building erection and fabrication practices than the prior art.
Another prior art system described in U.S. Pat. No. 8,683,758 to Christopoulos et al. describes a yielding fuse which includes elements that yield flexurally when a brace member moves in an axial direction, with the bracing assembly under either tensile or compressive loading conditions. The yielding fuse is connected co-axially with diagonal braces in a building structure. The non-yielding elements of this yielding fuse, the elastic portions, must be designed to resist axial, flexural, and shear forces. The end of each yielding arm is connected via bolts to a fabricated plate assembly, which is in turn connected to a structural gusset plate. At the other end of the fuse, the elastic portions are connected to the structural brace member. The Christopoulos et al. fuse requires substantial cast material to be provided in the elastic portions, and the design is fairly complex owing to the necessity to design for flexural, shear, and axial forces.
It is therefore an object of the invention to provide an improved yielding fuse.